Liquid level responsive shutoff valves are not new per se. See for example Babb 5,207,241. However, it has been found that the Babb valve as disclosed in the '241 Patent has a number of shortcomings which make it unsuitable for conditions in which the valve is exposed to relatively high or low ambient temperatures. Leakage can occur in and around the journal points of the valve spool under such extreme temperatures, particularly when high pressures are also attendant, and the valve spool itself may fail to completely close off the inlet passage in the valve body when the valve is supposed to be shutoff, thus causing over filling of the tank and spillage. The synthetic resinous spool of the Babb valve undergoes dimensional changes at a different rate than the main metallic body of the valve during exposure to extreme temperatures, thus causing the spool to bind up in the valve body and fail to open or close as the case may be. Resilient, elastomeric O-rings at opposite ends of the spool are intended to prevent leakage at such locations but actually become expanded under pressure and temperature extremes so as to abrade against adjacent stationary surfaces and undergo preliminary wear, leading to excessive leakage.
Furthermore, due to the fact that the valve spool has a crossbore that is axially aligned with the flow passage in the valve body when the spool is in its open position, the crossbore functions as a confining orifice through which the liquid must flow in order to reach the tank. As the spool is rotated toward a closed position in response to a float sensing the rising liquid level in the tank, the crossbore is moved progressively out of alignment with the passage to reduce the volume of liquid that can pass into the tank. As such closing rotation progresses, one inner wall of the crossbore rotates from a vertical disposition toward a horizontal orientation on the bottom side of the crossbore. This has the effect of introducing an obstruction or impingement surface in the path of fluid flow which produces a force tending to counteract the action of the float attempting to rotate the spool into its closed position. Consequently, while the float tries to close the valve and shutoff additional inflow, the high pressure inflow works against the interior impingement surface of the crossbore and tries to keep the valve open. If the supply pressure is high enough, the valve simply cannot close and the tank will overfill.
In an effort to reduce this countervailing force generated by the pressurized incoming flow, the Babb valve provides a relief notch in the lower, downstream edge of the crossbore so that the pressurized flow moving across such downstream edge when the spool has moved out of a fully open position exerts less counterrotating force against the spool than would otherwise be the case. However, it has been found that under certain conditions the spool with the relief notch in the crossbore closes prematurely. Moreover, the crossbore itself restricts the rate of volume flow of liquid through the valve such that the tank filling procedure takes longer than is desirable. It is also an important object to eliminate restrictions in the valve which limit the rate of volume flow, and to provide a construction which eliminates premature shutoff of the valve.